Ontos is Greek for "the thing." The M50 is a light, mobile, anti-tank system with six 106MM recoilless rifles. The Ontos was developed in the mid-1950s and designed for sea-to-land invasions. It was adopted by the U.S. Marine Corps and saw service in Vietnam. Its primary design flaw was that the recoilless rifles could only be reloaded from outside the vehicle.A Relic from the Vietnam WarThe Ontos system was designed for the mission of disarming and destroying enemy tanks with ease. Fitted with six M40A1C recoilless rifles and an M1919 Browning machine gun, the M50 was a formidable looking self-propelled armored vehicle. However, its flaws made it less intimidating.The guns on the Ontos may give it the appearance of something heinous, but the main issue with these guns was that they could not be loaded from inside the armored vehicle. This meant that loaders would have to load rounds from outside the vehicle, which made them prime targets for enemy fire and defeated the purpose of an armored vehicle. Certain environments and situations also rendered the mission of the M50 unnecessary, especially during its deployment in the jungles of Vietnam. North Vietnamese forces did not deploy many tanks, reducing the need for an anti-tank weapon system. The dense jungle did not make it any easier for mechanized artillery to deploy. The Ontos was therefore relegated to limited use as a field support weapon system, rather than a sea-to-land attack weapon.The design flaws were great enough that the U.S. Army cancelled their order. However, the Marine Corps committed to the 297 that were built, with the first vehicle accepted in 1956. When the systems was taken out of service in 1969, they were used by light infantry units in the Army. The remainder were all but removed entirely from service by 1970, and either scrapped or preserved in military museums. Fourteen M50s remain today, many of which are the M50A1 variant.